A large international Canadian-led clinical trial investigating a new way to prevent breast cancer in women at increased risk of developing the disease has found that the drug exemestane reduces this risk by 65 per cent compared with placebo. The results were presented today at the annual meeting of the American Society of Clinical Oncology and published online by the New England Journal of Medicine.

With funding from the Canadian Cancer Society, the NCIC Clinical Trials Group (CTG) led the trial which tested the drug exemestane, a member of a class of drugs called aromatase inhibitors. Drugs in this class suppress estrogen production, a key component in the development of some types of breast cancer.

“The results are extremely exciting because they have the potential to impact thousands of women,” says Dr Michael Wosnick, Vice-President of Research, Canadian Cancer Society. “We know that breast cancer takes a tremendous toll on Canadian women and their families. The results of this study offer an important new option to prevent this devastating disease in women who are at higher risk for it.”

The NCIC CTG MAP.3 trial followed more than 4,500 post-menopausal women from Canada, United States, Spain and France over a five-year period. All study participants were at increased risk for developing breast cancer. Risk factors include a woman’s age, her family history of breast cancer, her age at first menstrual period and her age at her first child’s birth. The study is the first randomized trial to determine if an aromatase inhibitor can be used to prevent breast cancer in healthy women.

At a median follow-up of three years, study researchers found that the group of women receiving exemestane had a 65% reduction in invasive cancers (11 invasive breast cancers in the exemestane group compared to 32 in the placebo group). Additionally, the researchers found fewer cases of pre-cancerous lesions in the group receiving exemestane.

The patient story

Vi Siemens and her three sisters have always been close and that relationship has been strengthened by their desire to do their part in preventing breast cancer. They have been participating in this landmark trial, sharing the daily bond of taking a pill and going for six-month check-ups.

The sisters lost their mother and two aunts to breast cancer so are only too aware of the risk they face. But they did not hesitate in their decision to join the trial.

“It wasn’t a tough decision for me to participate at all,” says Siemens. “All you have to do is look to the left or the right. Everybody knows somebody who’s had breast cancer in their family. Somebody has to test these drugs.”

Vi and her sisters know that they are part of something that can make a difference for themselves, for their family and for women everywhere.

“You’re thinking about the next generation,” says Vi. “You’d like to do something more than in just your own corner.”

This trial is just one of three conducted by the NCIC CTG and funded by the Canadian Cancer Society which this year has been selected as “Best of ASCO”. The other two studies include a landmark breast cancer prevention trial and one which found a more effective way to treat prostate cancer. Studies featured in Best of ASCO have the potential to change clinical practice around the world.

Breast cancer is the most common cancer among Canadian women (excluding non-melanoma skin cancer). In 2011, an estimated 23,400 women will be diagnosed with breast cancer and 5,100 will die of it.

The NCIC Clinical Trials Group is a national research program of the Canadian Cancer Society and receives programmatic funding, including for this trial, from the Canadian Cancer Society Research Institute. Additional support for the trial was provided by the Canadian Institutes for Health Research and Pfizer.

The Canadian Cancer Society is a national community-based organization of volunteers whose mission is the eradication of cancer and the enhancement of the quality of life of people living with cancer. When you want to know more about cancer, visit our website www.cancer.ca or call our toll-free, bilingual Cancer Information Service at 1 888 939-3333.